500 WiscG7isi7i Academy of Sciences, Aiis, and Letters. 



sucli a. process, but are just as likely to live for some time veg- 

 etativelv. The onlv case in which an excessive number of 

 nuclei need to lead to the assumption of fusion, would be to find 

 such a numerical excess in connection with the later stages of 

 cleavage, and this relation I have not found in any of my prep- 

 arations. It is quite probable that the large number of nuclei 

 is the result of a period of nuclear division just completed that 

 is to be followed by a pei-iod of cell growth. 



In cells in which cleavage is in progress the nuclei are very 

 frequently ai-ranged in pairs, the two nuclei of a pair often al- 

 most touching one another. (Figs. 25-30.) This arrange^ 

 ment is very clearly the result of the process of nuclear divi- 

 sion as I shall describe it farther on. Figure 21 shows a very 

 striking arrangement of the smaller irregular nuclei into groups 

 consisting of from two to eight individuals. How such an ar- 

 ran2:ement came about I have not been able to learn. The 

 forms of the nuclei might suggest that they have moved together 

 from various parts of the cell, but otlier evidence in support 

 of such a suggestion is entirely lacking. On the other hand 

 it is quite possible that each group repres.ents the product of 

 a series of successive nuclear divisions, and that the individual 

 nuclei will later be distributed more equally throughout tlie 

 cell. The fact that the dividing nuclei sometimes appear in 

 groups and that the daughter nuclei when first formed are fre- 

 quently quite irregular in shape, being thus identical in form 

 and structure with some of the grouped nuclei in the conditions 

 shovrn in Fig. 21, adds weight to this latter hypothesis. 



Owing to the small size of the nuclei it is impossible to make 

 out clearly all of the details of karyokinesis, but enough stages 

 stand out sharply to show that the process is essentially the 

 same as in the higher organisms. A verv distinct looselv coiled 

 thread is formed that apparently becomes segmented 

 into chromosomes (Figs. 7 and 8). In a few cases I have been 

 able TO make out quite clearly that there are ten segments; but 

 usually it was almost impossible to distinguish the' individual 

 chromosomes accurately enough to count them. When this 

 stage is reached the nuclear membrane has generally disap- 

 peared so that the peripheral parts of the chromosomes lie in im- 



